Reapply "Create an app snapshot of the Dart front end."

utils/kernel-service/kernel-service.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library runtime.tools.kernel_service;
 
 // This is an interface to the Dart Kernel parser and Kernel binary generator.
 //
 // It is used by the kernel-isolate to load Dart source code and generate
 // Kernel binary format.
 //
 // This is either invoked as the root script of the Kernel isolate when used
 // as a part of
 //
-//           dart --dfe=runtime/tools/kernel-service.dart ...
+//           dart --dfe=utils/kernel-service/kernel-service.dart ...
 //
 // invocation or it is invoked as a standalone script to perform batch mode
 // compilation requested via an HTTP interface
 //
-//           dart runtime/tools/kernel-service.dart --batch
+//           dart utils/kernel-service/kernel-service.dart --batch
 //
 // The port for the batch mode worker is controlled by DFE_WORKER_PORT
 // environment declarations (set by -DDFE_WORKER_PORT=... command line flag).
 // When not set (or set to 0) an ephemeral port returned by the OS is used
 // instead.
 //
 // When this script is used as a Kernel isolate root script and DFE_WORKER_PORT
 // is set to non-zero value then Kernel isolate will forward all compilation
 // requests it receives to the batch worker on the given port.
 //
@@ skipping 275 matching lines @@
         request.response.statusCode = HttpStatus.INTERNAL_SERVER_ERROR;
         request.response.headers.contentType = ContentType.TEXT;
         request.response.write(JSON.encode(result.toJson()));
         request.response.close();
       }
     });
     ProcessSignal.SIGTERM.watch().first.then((_) => server.close());
   });
 }
 
+train(String scriptUri) {
+  // TODO(28532): Enable on Windows.
+  if (Platform.isWindows) return;
+
+  var tag = 1;
+  var responsePort = new RawReceivePort();
+  responsePort.handler = (response) {
+    if (response[0] == tag) {
+      // Success.
+      responsePort.close();
+    } else if (response[0] == -tag) {
+      // Compilation error.
+      throw response[4];
+    } else {
+      throw "Unexpected response: $response";
+    }
+  };
+  var request = [tag, responsePort.sendPort, scriptUri];
+  _processLoadRequest(request);
+}
+
 main([args]) {
   if (args?.length == 1 && args[0] == '--batch') {
     startBatchServer();
+  } else if (args?.length == 2 && args[0] == '--train') {
+    // This entry point is used when creating an app snapshot. The argument
+    // provides a script to compile to warm-up generated code.
+    train(args[1]);
   } else {
     // Entry point for the Kernel isolate.
     return new RawReceivePort()..handler = _processLoadRequest;
   }
 }
 
 // This duplicates functionality from the Loader which we can't easily
 // access from here.
 Uri _findPackagesFile(Uri base) async {
   var dir = new File.fromUri(base).parent;
   while (true) {
     final packagesFile = dir.uri.resolve(".packages");
     if (await new File.fromUri(packagesFile).exists()) {
       return packagesFile;
     }
     if (dir.parent.path == dir.path) {
       break;
     }
     dir = dir.parent;
   }
   return null;
 }